Summary:
photosystem II (PSII) is a multiprotein complex contained within the thylakoid membranes of plants, algae and cyanobacteria, that utilizes solar energy for splitting water molecules. This reaction is the basis for oxygenic photosynthesis, transferring the electrons via an electron transfer chain that ends with a plastoquinone. This compound donates its electrons to the cytochrome b6f complex, which transfers them to plastocyanin, which in turn transfers the electrons to photosystem I, where they will eventually reduce NADP+ to NADPH. This provides reducing power for biosynthetic reactions and produces molecular oxygen as a by-product.

It had been shown that the PSII RC core complex of plants and cyanobacteria is dimeric, having a molecular mass of approx. 700 kDa.

There are many other intrinsic subunits in the complex, including a number of low-molecular mass proteins usually having a single transmembrane helix, which are rather featureless except for the Cytochrome b559 subunit α and PsbF proteins that provide histidine ligands for the high-potential haem of cytochrome b559.

The PSII RC core complex is attached to peripheral light-harvesting systems, which in cyanobacteria and red algae are composed of phycobilins, arranged within the extrinsically located phycobilisomes. There are usually 200-300 light harvesting pigment molecules serving one PSII RC [Barber06]. Most cyanobacteria have distinctly different pigment protein antenna systems, referred to as phycobilisomes [Hankamer01a]. These hemidiscoidal structures consist of rod-like arrays of water soluble phycobiliproteins, bound to the stromal surface of the PSII core by linker proteins. The rods are composed of discs that are hexamers of αβ monomers. Each αβ monomer covalently binds two (allophycocyanin) or three (phycocyanin) or more (phycoerythrin) open chain tetrapyrroles.

Excitation of the RC, via light absorption by chlorophylls in the antenna, drives electron transfer from the cluster of four chlorophylls (P680) that are bound to the D1/D2 reaction center complex to a pheophytin acceptor, resulting in a P680* radical. This results in electron transfer activities both downstream and upstream of P680*.

The downstream flow starts with the transfer of the P680* electron to a firmly bound plastoquinone, called QA, and on to a second plastoquinone, named QB. When QB is fully reduced and protonated to plastoquinol (PQH2), it diffuses from the QB-binding site into the lipid matrix of the membrane, where it travels to deliver its electron to the plastoquinol- plastocyanin reductase (better known as the cytochrome b6f complex), which in turn transfers the electron to plastocyanin that travels to photosystem I, where it delivers the electron.

The upstream flow consists of several electron transfers, that can be summarized as the transfer of electrons from two water moleules to the oxidized form of P680. The P680 radical cation (P680*) has a very high redox potential, estimated to be approx. 1.3 V, which is required to drive the water-splitting reaction. The oxidized form of P680 "pulls" an electron from a tyrosine residue (TyrZ) of the D1 protein, one of the protein components of the reaction center. An electron is then transferred to the tyrosine from a manganese atom which is part of a catalytic site called the oxygen-evolving center (OEC). This site is composed of a cluster of four Mn and one Ca2+ ion. Each time P680 is excited by absorbing a photon, the electron transfer results in the removal of an electron from one of the Mn atoms. When four such cycles have occurred, the OEC is ready to receive the four electrons that are involved in the splitting of two water molecules to form dioxygen.

When the water molecules are split and their electrons are transferred to the OEC, two additional "by-products" are formed - molecular oxygen and protons. This accumulation of protons in the thylakoid lumen results in the build-up of a proton gradient across the memberane, which is utilized by ATP synthase to generate ATP.